The present invention relates to telephone circuitry and in particular, to telephone circuitry for allowing the proper party on a party telephone line to receive an incoming message. In particular, the present invention relates to a party telephone line selection apparatus and method, which is responsive to the frequency of an incoming ringing signal to thereby determine if the incoming call is for a particular party of a group of party line users.
In party telephone lines, a number of parties share a common telephone line, for example, four. Various systems are in use for determining the party to which a call is directed. Due to the recent deregulation of the telephone industry, the older systems in which the ringer in each telephone was individually tuned to a specific frequency or was sensitive to ringing signals having a specific polarity, are no longer as useful, because individual customers may now purchase their own phones from other than their telephone company. Accordingly, an unmodified standard telephone which is purchased by a customer will not contain the proper circuitry for party line use. Therefore, it is necessary to provide special circuitry which can be connected into the telephone line itself for directing incoming calls to the proper party on a party line.
As indicated, in prior art systems using frequency responsive circuitry, a special circuit is installed in the individual telephone. Two common systems include the "deciharmonic" system, in which frequencies of 20, 30, 40, 50, 60, etc. Hz are transmitted along the party telephone line as the ringing signals. A special circuit tuned to the particular frequency may be located in the phone. In another system, called the "harmonic" system, frequencies of 16.67, 25, 33.33, 42, 54, 66.67, etc. Hz are utilized. Of course, additional frequencies can also be provided depending upon the number of party line users. In a typical application, four users are installed on a single party line. If a standard telephone is installed on a party line, the standard telephone will respond to most frequencies between about 16 and 33 Hz, and will not respond to frequencies above about 33 Hz. Accordingly, a standard telephone will either respond to all calls if the ringing signal is between 16 and 33 Hz, even if the calls are not directed to that party or will not respond to all calls if the ringing frequency is above 33 Hz, even if the calls are directed to that party.